1. Field of the Invention
The present invention relates to an ophthalmologic image taking apparatus that picks up an image of an eye to be examined in an ophthalmologic clinic and the like.
2. Related Background Art
Up to now, there has been known an ophthalmologic image taking apparatus in which an image of an eye to be examined is picked up using an image pickup device represented by a CCD and converted into a video signal, thereby conducting an observation of the eye to be examined and image taking thereof. In such an ophthalmologic image taking apparatus, an output signal from the image pickup device is electrically amplified in order to conduct control by which the image of the eye to be examined always becomes a suitable video signal.
In the case where infrared fluorescent image taking is conducted, an eye fundus cannot be observed through an optical finder. Therefore, two kinds of cameras for observation and image taking are prepared and used with gains thereof being set to be different from each other. However, using two television cameras causes the ophthalmologic image taking apparatus to become complicated and further increasing a cost thereof.
In order to solve the above-mentioned problem, an ophthalmologic image taking apparatus that conducts the observation of the eye to be examined and the image pickup thereof using a single image pickup device is disclosed in Japanese Patent Application Laid-Open Nos. H9-262211 and H10-234671, or the like.
However, there is no description with respect to gain setting in Japanese Patent Application Laid-Open No. H9-262211. In addition, in the ophthalmologic image taking apparatus disclosed in Japanese Patent Application Laid-Open No. H10-234671, an ND filter is changed in accordance with an image taking mode to change the light intensity for image taking in order to conduct control by which an image of the eye to be examined becomes an image of a suitable video signal.
In the case of an observation using visible fluorescence or infrared fluorescence, the brightness of the eye to be examined changes according to a circulating state of a fluorescer. In particular, because the change in the brightness of the eye to be examined is remarkable in an initial image, auto gain control (AGC) is operated. Therefore, even if the brightness of the eye to be examined, that is, even if the light intensity for observation changes, it is desirable to conduct the control such that the image of the eye to be examined always becomes an image of a suitable video signal.
On the other hand, because a light emission time of an image taking light source used in the case of image taking is a short time of several milliseconds, even if the AGC is executed, the change in the brightness of the eye to be examined cannot be attained accordingly. Therefore, instead of the gain obtained by the AGC, a fixed gain is used in image taking and the light intensity for image taking is adjusted according to the brightness of the eye to be examined, thereby making it possible to conduct the control by which the image of the eye to be examined becomes an image of a suitable video signal.
However, in the case where the AGC is switched to the fixed gain control, a predetermined time is required to stabilize the gain. Such predetermined time is caused by, in the case where an apparatus main body and a television camera unit are used in combination and switching between the gain obtained by the AGC and the fixed gain is controlled using a communication port such as RS-232C, a communication time and a response time of a gain circuit. If the image taking light source emits light to conduct image taking with a state in which the gain control is not completely switched, the brightness is saturated or an image becomes dark. That is, a suitable video signal is not captured.
In contrast to this, even in the case where the fixed gain is switched to the gain obtained by the AGC, a predetermined time is required to stabilize the gain. In this case, in addition to a cause of a time delay in the case where the gain obtained by the AGC is switched to the fixed gain, a delay of a time corresponding to at least a frame or a field is caused because the gain is determined based on a video signal of the picked up image of the eye to be examined. Therefore, a time is further required to converge the gain. If moving image recording of the eye to be examined is conducted under a state in which the gain control is not completely switched, the brightness is saturated or an image becomes dark. That is, a suitable video signal is not captured.